Origin and biological properties of a new BALB/c mouse sarcoma virus

A focus-forming virus previously isolated from a BALB/c mouse hemangiosarcoma has been shown to be replication defective. Analysis of individual BALB/c mouse sarcoma virus (BALB-MSV) nonproducer transformants for expression of helper virus-coded proteins revealed genetically stable variants that expressed two, three, or all four gag gene products in the absence of detectable helper viral env gene expression. The type-specific antigenic determinants of helper viral proteins encoded by the BALB-MSV genome and by the B-tropic virus isolated from the BALB-MSV stock were demonstrated to be indistinguishable from those of BALB:virus-1, a known endogenous virus of BALB/c cells. These findings imply that a BALB/c endogenous virus was involved in the generation of BALB-MSV. By the same immunological approach, the presence of at least a portion of the Moloney-MuLV gag gene has been identified in two other transforming viruses--Moloney-MSV and Abelson lymphosarcoma virus--previously isolated from the BALB/c strain. The tissue culture properties of cells transformed by these defective viruses were also shown to be distinguishable. These findings indicate that transforming virus isolates of the same inbred strain differ in their transforming activities as well as in the helper viral sequences stably associated with their genomes.